Based on a quick internet scan, there are a wealth of hits re Volvo door lock problems - almost all of them related to unlocking problems - mine is different. I recently acquired a 2009 V70 3.2. The remote "unlock" function is fine as is the internal control but, the passenger side rear door lock won't lock with the remote or the door control. If I open the door and push down the lock, it is locked and will open with the remote but although unlocked, the button does not move as far up as the buttons on the other doors. When I use the remote or door control to lock the doors, it moves down a little but is not locked.

Any suggestions short of opening up the whole door panel?

Thanks

 

Sounds like a Faulty actuator to me.

 

Have had an identical problem but in a V70 mk2, turned out that the lock wire was stucked by the materials i had used for sound proofing so i could only lock it manually. Not sure though how it all looks like in the mk3 but you could give it a try.